ORNL, UTK team maps the nuclear landscape

This image represents the nuclear landscape, with isotopes arranged by an increasing number of protons (up) and neutrons (right). The dark blue blocks represent stable isotopes. The lighter blue blocks are unstable isotopes that have been observed. The gray blocks are bound isotopes that have not been observed. Nuclear existence ends at the drip lines (orange clouds), where there is no longer enough binding energy to prevent the last nucleons from dripping off (floating blocks). Image: Andy Sproles, Oak Ridge National Laboratory

An Oak Ridge
National Laboratory and University
of Tennessee team has
used the U.S. Department of Energy's Jaguar supercomputer to calculate the
number of isotopes allowed by the laws of physics. The team, led by Witek
Nazarewicz, used a quantum approach known as density functional theory,
applying it independently to six leading models of the nuclear interaction to
determine that there are about 7,000 possible combinations of protons and
neutrons allowed in bound nuclei with up to 120 protons (a hypothetical element
called "unbinilium"). The team's results are presented in Nature.

Most of these
nuclei have not been observed experimentally.

"They are
bound, meaning they do not spit out protons or neutrons," Nazarewicz
explained. "But they are radioactive—they are short-lived, because there
are other processes, such as beta decay, that can give rise to
transmutations."

Of the total,
about 3,000 have been seen in nature or produced in nuclear physics
laboratories. The others are created in massive stars or in violent stellar
explosions.

The computations
allowed the team to identify the nuclear drip lines that mark the borders of
nuclear existence. For each number of protons in a nucleus, there is a limit to
how many neutrons are allowed. For example, a helium nucleus, which contains
two protons, can hold no more than six neutrons. If another neutron is added to
the nucleus, it will simply "drip" off. Likewise, there is a limit to
the number of protons that can be added to a nucleus with a given number of
neutrons. Placement of the drip lines for heavier elements is based on
theoretical predictions extrapolated far from experimental data and is,
therefore, uncertain.

The closer an
isotope is to one of these drip lines the faster it decays into more stable
forms. Particle accelerators have been unable to identify most of these exotic
isotopes, especially those approaching the neutron drip line, because they are
impossible to produce using current combinations of beams and targets. In fact,
said Nazarewicz, all radioactive isotopes decay until they are transformed into
one of 288 isotopes that form the so-called "valley of stability."
These stable isotopes have half-lives longer than the expected lifetime of the
solar system (about 4.6 billion years).

Earlier
estimates of the nuclear landscape varied from as few as 5,000 to as many as
12,000 possible nuclei, Nazarewicz noted. He said his team's calculations were
based on the microscopic forces that cause neutrons and protons to cluster into
nuclei, adding that results from the six separate models were surprisingly
consistent. By using several models, theorists were able for the first time to
quantify uncertainties of predicted drip lines.

Because most of
these nuclei are beyond our experimental reach, he explained, models must
conform to known nuclei in a way that allows researchers to extrapolate results
for exotic nuclei. Insight on the nature of most exotic nuclei must be extrapolated
from models, he said.

"This is
not a young field," Nazarewicz noted. "Over the years we've tried to
improve the models of the nucleus to include more and more knowledge and
insights. We are building a nuclear model based on the best theoretical input
guided by the best experimental data."

The calculations
themselves were massive, with each set of nuclei taking about two hours to
calculate on the 244,256-processor Jaguar system. Nazarewicz noted that each of
these runs needed to include about 250,000 possible nuclear configurations.

"Such
calculation would not be possible two to three years ago," he said.
"Jaguar has provided a unique opportunity for nuclear theory."

Nazarewicz noted
that this work, supported by DOE's Office of Science—which also supports the
Jaguar supercomputer—and by the Academy of Finland, has both existential value,
helping us to get a better understanding of the evolution of the universe, and
potential practical applications.

"We are not
doing nuclear physics just to see whether you can get 7,000 species," he
explained. "There are various nuclei that we can use to our advantage,
eventually. Those we call 'designer nuclei.'"

Among these
valuable nuclei are iron-45, a collection of 26 protons and 19 neutrons, which
may help us understand superconductivity between protons; a pear-shaped
radium-225, with 88 protons and 137 neutrons, which will help us understand why
there is more matter than antimatter in the universe; and terbium-149, with 65
protons and 84 neutrons, which has shown an ability to attach to antibodies and
irradiate cancer cells without affecting healthy cells.

"They have
done experiments on mice and now humans in which they would look at the
effectiveness of this treatment," he said. "This treatment is called
an 'alpha knife.'